1. Field of the Invention
The present invention relates generally to a CDMA (code division multiple access) mobile radio communication system. In particular, the present invention relates to a call control method in a base station of a CDMA mobile radio communication system which can maintain the forward load in a sector below a predetermined level by estimating the forward powers of activated traffic channels allocated to the existing sector and then by calculating the forward sector excess capacity when a respective base station of the mobile radio communication system controls the originating and terminating calls.
2. Description of the Related Art
A base station of a conventional mobile radio communication system is composed of a selector/vocoder module for performing a voice process and allocating forward traffic channel gains, a plurality of traffic channel elements for calculating the respective allocated powers, and a base station processor (BSP) for calculating the forward powers of the sector using the allocated power values received from the traffic channel elements and controlling the sector capacity accordingly.
Generally, it is important that the allocation of channels to new calls requested by new mobile units should be performed with the speech quality of the existing mobile units in use guaranteed if the new mobile units continuously request calls in a state that the existing mobile units are in use.
The maximum capacity of the forward link, whereby a specified speech quality can be guaranteed with respect to the calls being served as a plurality of mobile units in a sector are maintained in a serviceable state that they can process terminating and originating calls, is called a maximum forward sector capacity. If a load exceeding the maximum forward sector capacity exists in the sector, a pilot signal in the sector may disappear due to the traffic signal. Also, since the frame error ratio of the forward sector is increased, deterioration of the speech quality and the call release phenomenon may occur. The sector load which affects the forward sector capacity is increased and decreased by the number of the allocated calls, speech activity, forward control subchannel, etc.
A forward sector capacity control technique according to the number of calls and a high-power amplifier overload control technique 7 are known as conventional techniques relating to the sector capacity control.
The former is a method of controlling the sector capacity based on the number of activated calls which is being served in the sector. The latter is a method of controlling the sector capacity by monitoring the effective radiated power of the base station and preventing the malfunction of the high-power amplifier caused by overload, which is the base station output amplifier, based on its effective radiated power.
The conventional method of controlling the sector capacity according to the number of calls will now be explained in detail. According to this method, a fixed service block threshold value (the number of calls) is carried out by an experiment considering the sector output, propagation environments, etc. The service block threshold value may be classified into a call attempt block threshold value, a handoff attempt block threshold value, etc. The base station compares the total number of calls being served in the sector with the call attempt block threshold value whenever it receives the call attempt request from a mobile unit in the sector. At this time, if the total number of calls is smaller than the call attempt block threshold value, it allocates the call, while if the total number of calls is the same as or larger than the call attempt block threshold value, it puts the call attempt request in a standby state without allocating the call. Meanwhile, if a handoff request from an adjacent sector to the present sector is received, the base station compares the total number of calls being served in the sector with the handoff attempt block threshold value. At this time, if the total number of calls is smaller than the handoff attempt block threshold value, it allocates the handoff call, while if the total number of calls is the same as or larger than the handoff attempt block threshold value, it puts the handoff attempt request in a standby state without allocating the handoff call. The call attempt block and the handoff attempt block are released when the total number of calls becomes smaller than the respective threshold values by the normal completion of the calls in the sector, and at this time, the base station allows the call attempt request and the handoff attempt request of the sector.
However, according to the conventional sector capacity control method according to the number of calls, since the respective sectors have different propagation environments from one another, and the maximum numbers of calls serviceable in the respective sectors are different accordingly, the service block threshold values according to the number of calls in the respective sectors are also different from one another. Also, since the number of subscriber's calls serviceable in the sectors are not absolute, they are insufficient to be used as the basis of judgement on the sector capacity control.
Meanwhile, the conventional method of controlling the overload of the high-power amplifier will now be explained. According to this method, a specified service block threshold value at the final output terminal is determined considering the maximum output of the high-power amplifier which is the base station output amplifier. The service block threshold value may be classified into a call attempt block threshold value, a handoff attempt block threshold value, etc. The base station always monitors the effective radiation output of the sector at the final output terminal, and compares the output of the sector with the call attempt block threshold value and the handoff attempt block threshold value, respectively. The base station allows or bars the call attempt and the handoff attempt according to the result of comparison.
However, the conventional high-power amplifier overload control method has the drawback that the output of the sector is abruptly reduced in case that the high-power amplifier is overloaded and operates abnormally, and this causes the calls being served in the sector to be released. In other words, since the object of the overload control function performed by the high-power amplifier is to protect the high-power amplifier rather than to control the sector capacity, consideration is not given to the speech quality of the sector, the intensity of the pilot signal, the abnormal call release caused by the increase of the sector capacity, etc. Accordingly, the sector capacity may be in saturation before the high-power amplifier is overloaded.
As described above, the decrease of the sector capacity due to the increase of the subscribers causes the estimation and the proper control of the speech quality of the sector not to be performed. In the worst case, the pilot signal of the sector may disappear, and thus the sector may be in an unserviceable state. This results in release of the calls being served, thereby deteriorating the reliability of the whole communication system.